The present inventions relate to methods, systems and apparatuses for performing measurement pertaining to magnetic field, more particularly to such methods, systems and apparatuses for measuring a magnetic field at the surface of a ferromagnetic material.
Ships and submarines are constructed of ferromagnetic materials which produce magnetic field signatures, making them detectable and vulnerable to magnetic influence sea mines and detectable by airborne magnetic anomaly detection (MAD) and underwater electromagnetic surveillance systems.
To reduce the magnetic field signature of ships and submarines, coils are wrapped around the ferromagnetic hull, and fields produced which reduce the vessel""s signature. In order to control the coil currents, a degaussing (DG) system must have sensors which accurately measure the signature-related magnetic fields, and control algorithms to extrapolate the spatially measured field values to regions under the ship, and adjust the coil currents to minimize the signature amplitude.
It is useful to measure magnetic fields near the hull of naval ships and submarines, so that such measured magnetic fields can be used to control advanced degaussing systems. A large number of xe2x80x9cpointxe2x80x9d sensors are presently employed, but they are expensive and not capable of satisfying the need for measuring fields at all points along the circumference of a ship or submarine hull. It is important to measure these fields produced by local hull anomalies (welds, stresses, bulkheads, etc.) and material inhomogeneities at many locations, for more effective control of the ship""s degaussing system. Ideally, by measuring the surface magnetic fields all over the hull (and thereby continuously monitoring the magnetic state of a ship or submarine hull), the magnetic field signature of the ship can be adjusted and maintained at a low level using an advanced degaussing system such as the U.S. Navy""s Advanced Closed Loop Degaussing System, thereby maldng a ship less vulnerable to sea mine magnetic influence fuzes.
Advanced degaussing systems require accurate and spatially distributed magnetic field measurements around the ship, so that ship mathematical model algorithms can precisely control magnetic field signatures below the ship. Some of the problems associated with measuring these fields include: large spatial gradient magnetic fields; local magnetic anomalies; induced magnetic fields caused by heading changes; and, permanent magnetization changes due to pressure-induced hull stresses. Such measurements have been made using traditional fluxgate magnetometers, short baseline gradiometers, etc.
In some cases, there are large spatial magnetic field gradients, close to the hull, which are produced by local hull anomalies (e.g., welds, stresses, bulkheads, etc.) and material inhomogeneities. xe2x80x9cPointxe2x80x9d triaxial fluxgate magnetometers and gradiometers are presently used to measure these spatial gradients; however, because of these local effects, field measurements at many locations may not be useful for controlling the shipboard degaussing system.
Fluxgate magnetometers measure the magnetic field intensity using a variety of transducer cores which, normally, are considered to be small xe2x80x9cpointxe2x80x9d field sensors (typically, about one to two inches in length). More generally, fluxgate, fiber-optic and other magnetic field sensitive transducer phenomena measure the magnetic field intensity using a variety of transducer cores which are normally considered point field measurements (wherein the transducers are typically about one to two inches in length).
A ship or submarine with a ferromagnetic hull produces a magnetic field signature which is dependent on the hull material magnetic characteristics, it""s geometry in the earth""s magnetic field, and stresses which are applied to the hull. Present degaussing systems sense the magnetic fields relatively close to the hull, and adjust the degaussing coil currents to minimize the fields at a distance below the vessel which can be sensed by magnetic influence sea mines. The ferromagnetic hull material""s characteristics are used in complex ship models which are able to predict a vessels magnetic field signature below the ship. However, the characteristics may change significantly with respect to stress, heading, and time.
Ferromagnetic material sample characteristics are presently measured using ASTM xe2x80x9cStandard Methods of Testing Magnetic Materials, which include DC fluxmeter, and alternating current techniques; see ASM Standard Methods of Testing Magnetic Materials, A34-70, American National Standards Institute, Part 8, incorporated herein by reference. Other techniques include balance permeameters for feebly magnetic materials, and portable permeameters, sometimes used as xe2x80x9cmagnaflux probexe2x80x9d for non-destructive testing of structural materials; see Sery, R. S., Permeameter Development and Use for Measuring Magnetic Permeability of SSN and High Strength Steels, NSWC TR 80-347, Oct. 1, 1980, Naval Surface Weapons Center, White Oak, Md., incorporated herein by reference.
Other pertinent background information is provided by the following papers, each of which is hereby incorporated herein by reference: Lenz, J. E., xe2x80x9cA Review of Magnetic Sensors,xe2x80x9d IEEE Proceedings, Vol. 78, No. 6, June 1990;Gordon, D. I., R. E. Brown and J. F. Haben, xe2x80x9cMethods for Measuring the Magnetic Field,xe2x80x9d IEEE Trans. Mag., Vol. Mag-8, No. 1, March 1972; Gordon, D. I. and R. E. Brown, xe2x80x9cRecent Advances in Fluxgate Magnetometry,xe2x80x9d IEEE Trans. Mag., Vol. Mag-8, No. 1, March 1972; Gordon, D. I., R. H. Lundsten, R. A. Chiarodo, H. H. Helms, xe2x80x9cA Fluxgate Sensor of High Stability for Low Field Magnetometry,xe2x80x9d IEEE Transactions on Magnetics, vol. MAG-4, 1968, pp 379-401; Acuna, M. H., xe2x80x9cFluxgate Magnetometers for Outer Planets Exploration,xe2x80x9d IEEE Transactions on Magnetics, vol. MAG-10, 1974, pp 519-23.
In view of the foregoing, it is an object of the present invention to provide method, apparatus and system for measuring magnetic characteristics of a ferromagnetic material such as that of a ship""s hull.
It is another object of the present invention to provide method, apparatus and system for continuously measuring same, for use in association with a magnetic control system such as a ship degaussing system.
All magnetic materials can be characterized by a hysteresis curve, which is a two dimensional plot of Induction (B in weber/m2=104 gauss) versus Magnetic Field intensity (H in ampere/meter=0.01256 Oersted). The ratio of B/H is defined as the magnetic permeability of the material (weber/m-amp=henry/meter=newton/amp2 or 1 hy/m=79.6xc3x97103 gauss/oersted), and varies non-linearly with respect to field amplitude, and mechanical stress.
At the surface of a ship or submarine hull or any ferromagnetic material, the normal component of the Induction (B) and the transverse component of Field Intensity (H) are each continuous across the surface boundary. The fields at the surface are dependent on the bulk material magnetic properties (permeability), which are dependent on the ambient magnetic field, the stress on the material which changes the characteristics of the magnetic material, and other local effects.
The xe2x80x9cFerromagnetic Surface Magnetic Field Sensorxe2x80x9d (xe2x80x9cFSMFSxe2x80x9d) in accordance with the present invention features measurement of magnetic field at the surface of a ferromagnetic material (e.g., at the surface of a ship""s hull) by measuring either or both the transverse H field and the normal B (Induction), using the ferromagnetic properties of the material as part of the sensor transducer. This invention advantageously senses magnetic characteristics of ferromagnetic material while obviating the need to alter such material.
The present invention provides a fluxgate device for sensing the transverse component of the magnetic field intensity H at a surface area of a ferromagnetic entity. The inventive device comprises a magnetic core, a drive winding and two sense windings. The magnetic core generally describes a three-dimensional xe2x80x9cExe2x80x9d shape. The magnetic core including four portions. The four portions are a base portion and three leg portions each projecting from the base portion. The three leg portions are a first end leg portion, a second end leg portion and a middle leg portion. The middle leg portion is approximately equidistantly interposed between the first end leg portion and the second end leg portion. The first end leg portion, the second end leg portion and the middle leg portion each have a leg end surface for being situated adjacent the surface area of the ferromagnetic entity when the device is positioned with respect to the ferromagnetic entity. The drive winding is wound over the middle leg portion. The two sense windings are a first sense winding and a second sense winding. The first sense winding is wound over the base portion between the first end leg portion and the middle leg portion. The second sense winding is wound over the base portion between the second end leg portion and the middle leg portion. Typically, the device further comprises three calibration windings. The three calibration windings are a first calibraton winding, a second calibration winding and a third calibration winding. The first calibration winding is wound over the first end leg portion. The second calibration winding is wound over the middle leg portion. The third calibration winding is wound over the second end leg portion.
The present invention also provides a fluxgate device for sensing the normal component of the magnetic induction B at a surface area of a ferromagnetic entity. The device comprises a magnetic core, a drive winding and a sense winding. The magnetic core generally describes a semi-open coaxial double-cylinder shape. The magnetic core includes an approximately cylindrical bucket-shaped portion and an approximately cylindrical solid portion. The approximately cylindrical solid portion has a smaller diameter than has the approximately cylindrical bucket-shaped portion. The approximately cylindrical bucket-shaped portion has an annular end surface. The approximately cylindrical solid portion has a continuous circular end surface. The annular end surface and the continuous circular end surface are for being situated adjacent the surface area of the ferromagnetic entity when the device is positioned with respect to the ferromagnetic entity. The drive winding is wound over the approximately cylindrical solid portion. The sense winding is wound over the approximately cylindrical solid portion. Typically, the device further comprises a calibration winding which is wound over the approximately cylindrical solid portion.
The purpose of the present invention, in the context of the U.S. Navy""s effort, is to continuously measure magnetic characteristics of the material of a ship""s hull at many locations, and to supply the measured data to the ship degaussing system""s model-based control algorithms. The present invention""s U.S. Navy prototype FSMFS is designed to measure a ferromagnetic hull""s magnetic characteristics continuously. According to inventive principles, the B and H values of the surface properties of the hull can be determined by using the hull material itself as part of the transducer element. The inventive methodology dynamically measures, in real time, any change in hull magnetic characteristics including permanent magnetism, as well as induced magnetism and magnetization produced by hull stress.
Thus provided by the present invention is a type of ferromagnetic material (e.g., hull material) xe2x80x9cpermeameter.xe2x80x9d The invention""s FSMFS can use either crystalline or amorphous magnetic materials in the transducer core. The U.S. Navy""s prototype FSMFS sensor uses modified electronics which the U.S. Navy developed for its prototype IFM sensor.
Related to (but distinguishable from) the inventive FSMFS is the inventive xe2x80x9cIntegrating Fluxgate Magnetometerxe2x80x9d (IFM) which is disclosed by the aforementioned U.S. Pat. No. 6,278,272 B1. A typical inventive Integrating Fluxgate Magnetometer (IFM) is a fluxgate magnetometer having a rigid transducer core which is configured as a long xe2x80x9crace trackxe2x80x9d in order to integrate large component gradient magnetic fields near a ferromagnetic entity, e.g., a ship hull or a large piece of machinery. A typical inventive IFM: (i) measures magnetic fields over the length of its elongated transducer element (e.g., the 30 cm length of an inventive prototype tested by the U.S. Navy), and (ii) spatially integrates the component field amplitudes.
Also related to (but distinguishable from) the inventive FSMFS is the inventive xe2x80x9cSpatially Integrating Magnetometerxe2x80x9d (SIM) which is disclosed by the aforementioned U.S. nonprovisional patent application Ser. No. 09/517,560. A typical Spatially Integrating Magnetometer (SIM) measures the magnetic field at discrete distributed points, or summation of all field components, along a xe2x80x9clinear,xe2x80x9d flexible transducer element. According to many inventive SIM embodiments, a spatially integrating transducer magnetometer measures the magnetic field components (tangential and normal) over a very long linear region, at discrete points, and integrates component field values (the sum of the field component amplitudes) over the length of the spatially integrating tranducer magnetometer""s sensor element. A typical inventive SIM: (i) measures magnetic field amplitude components over a very long linear region, at discrete points, and (ii) integrates these component field values (the sum of the field component amplitudes) over the length of the transducer element.
Also related to (but distinguishable from) the inventive FSMFS is the inventive xe2x80x9cStanding Wave Magnetometerxe2x80x9d (SWM) which is disclosed by the aforementioned U.S. Pat. No. 6,344,743 B1. In accordance with many embodiments of the inventive SWM, a methodology is provided for determining the distribution of a magnetic field in a spatial sector. According to a typical inventive SWM, a magnetic field amplitude value is measured at each of a plurality of points in the sector, wherein the means for measuring is characterized by a length which is defined by the points. Alternating current is applied at a high frequency having an associated wavelength which corresponds to a multiple of the length. The applied alternating current is conducted so as to establish a standing wave along the length. The measured magnetic field amplitude values are processed; this processing includes performing, over the multiple of the length, Fourier analysis based on a harmonic bias function which results from the standing wave.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
Incorporated herein by reference is the following technical report: Scarzello, John F. and Edward C. O""Keefe, xe2x80x9cDevelopment of Shipboard Magnetic Sensors for Degaussing System Controllers,xe2x80x9d NSWCCD-TR-98/011, Jun. 30, 1998, Machinery Research and Development Directorate Research and Development Report, Naval Surface Warfare Center, Carderock Division, West Bethesda, Md. 20817-5700. See, especially, Chapter 6 of this report. This report includes 93 pages, including 43 pages of drawings.